The invention relates to a frequency converter and a method for frequency conversion. It is related particularly, but not exclusively, to wireless mobile stations.
The continuous demand for more compact and inexpensive mobile phones has led to the search for alternative radio architectures to replace the traditional superheterodyne architecture. Architectures having direct conversion receivers and low-IF (Intermediate Frequency) receivers have been proposed. Unfortunately, the alternative receiver architectures have drawbacks. Direct conversion receivers are sensitive to DC offset and with low-IF receivers it is difficult to achieve sufficient suppression of image frequencies. Problems also exist with single sideband transmitters. Poor image suppression causes carrier leakage and signal leakage to the image frequencies. In all these cases, insufficient matching between two mixer branches will lessen the performance of a receiver, a transmitter or a mixer.
There are several sources of DC offset in a direct conversion receiver. One harmful DC offset component is proportional to the input power, and will make the resulting DC level change corresponding to changes in the incoming power level. This phenomenon is called AM (Amplitude Modulation) detection. AM detection is harmful in TDMA (Time Division Multiple Access) systems and systems using linear modulation schemes. The reason for this power detection lies in the signal squaring behaviour in certain functional blocks of a receiver. A mixer often presents most problems, because it must be capable of handling all the incoming power although it is DC coupled with the baseband circuit blocks.
The traditional way to avoid AM detection is by the use of balanced frequency converters or mixers. FIG. 1 shows an example of a single balanced frequency converter 10 according to the prior art, in order to illustrate the operating principle of typical single balanced frequency converters. The converter 10 receives an incoming signal, filters out non-desired frequencies to give a desired frequency band, generates two parallel, out-of-phase mixed signals by mixing the desired band with out-of-phase local oscillator (LO) signals, combines both mixed out of phase signals to a single signal and, finally, post-selection filters them.
The converter 10 comprises a pre-selection filter 13 having an input port for receiving an incoming signal x. The pre-selection filter has an output port for providing a filtered incoming signal xxe2x80x2 having the desired frequency band. This filtered incoming signal xxe2x80x2 is then to be mixed with LO signals of opposite phases by two mixing elements MIX1A and MIX1B. The two mixing elements are located in parallel branches. A first mixing element MIX1A has an input port for the filtered incoming signal xxe2x80x2 and another input port for an in-phase LO signal. A second mixing element MIX1B has also an input port for the filtered incoming signal xxe2x80x2 and another input port for an anti-phase LO signal.
The in-phase and anti-phase LO signals originate from an LO signal z, which is coupled to an input port of a phase shifter unit 11. The LO signal is of a frequency fLO. The phase shifter unit 11 produces an in-phase output 11A and a 180 degrees (xcfx80) phase-shifted anti-phase output 11B from the LO signal z. The in-phase output 11A is coupled to the LO input port of the first mixing element MIX1A to produce a first mixed signal and the anti-phase output 11B is coupled to the LO input port of the second mixing element MIX1B to produce a second mixed signal.
The outputs of the first and second mixing elements MIX1A and MIX1B, which are the first mixed signal and, respectively the second mixed signal, are coupled to first and second input nodes 12A and 12B of a combiner 12. The first mixed signal can be described by a formula K1xxe2x80x2xc2x7z+k2xc2x7xxe2x80x22, wherein k1 and k2 are the constants for the two mixing products. Correspondingly, the second mixed signal can be described by a formula xe2x88x92k3xxe2x80x2xc2x7z+k4xxe2x80x22, wherein xe2x88x92k3 and k4 again are the constants for the mixing products.
The first and second mixed signals represent two parallel branches, corresponding to two opposite phases of the LO signal. The combiner 12 subtracts the second mixed signal from the first mixed signal creating a combined signal. The combined signal is (k1+k3)xxe2x80x2xc2x7z+(k2xe2x88x92k4)xc2x7xxe2x80x22. The latter term of the formula, the square term, is of great importance. If the characteristics k2 and k4 of the two mixing elements are identical, the output of the converter 10 contains only the desired product of the input signals. However, if there is a difference between the characteristics k2 and k4 of the mixing elements MIX1A and MIX1B, a squared term proportional to the difference k2xe2x88x92k4 is produced in combiner 12 and is included into the combined signal. The degree to which the two branches of a balanced frequency converter must be matched depends on the system to be constructed. For example, to meet the GSM specifications, the mixing elements need to be very closely matched indeed: k1/(k2xe2x88x92k4)≳1000.
The combiner 12 has an output for providing the combined signal to a post-selection filter 14, which filters out a desired output frequency band.
In FIG. 1 the mixing elements MIX1A and MIX1B have been drawn separately for their better illustration, although typically they are integrated within a single mixing block MIX1.
According to a first aspect of the invention there is provided a method for converting frequency of an electrical signal, comprising the steps of:
receiving an input signal;
generating a first local signal;
generating a second local signal that is in a different phase than the first local signal;
mixing said input signal with the first local signal to obtain a first product signal;
mixing said input signal with the second local signal to obtain a second product signal; and
combining said first and second product signals together, wherein
said mixing of said input signal with said first local signal occurs in one time period;and
said mixing of said input signal with said second local signal occurs in another time period.
According to a second aspect of the invention there is provided a frequency converter for converting frequency of an electrical signal, comprising:
an input for receiving an input signal;
a first signal source for providing a first local signal;
a second signal source for providing a second local signal that is in a different phase than the first local signal;
a first mixing element for mixing said input signal with the first local signal to obtain a first product signal;
a second mixing element for mixing said input signal with the second local signal to obtain a second product signal; and
a combiner for combining said first and second product signals together, wherein
said first mixing element is arranged to perform the mixing of said input signal with said first local signal in one time period; and
said second mixing element is adapted to perform said mixing of said input signal with said second local signal occurs in another time period.
Preferably, the frequency converter is integrated into a mobile radio device. Such a mobile radio device can avoid at least most of the AM detection.
Preferably, the frequency converter is integrated into a radio network element. Such a radio network element can avoid at least most of the AM detection.
Preferably, the frequency converter is integrated into a radio network. Such a radio network can avoid at least most of the AM detection.
The invention can be used in a number of different implementations comprising mobile telephones and data terminals, radio transceivers of various digital and analogue radio circuits and devices, including radiotelephone terminals and base stations.